170 Handbook of Nutraceuticals and Functional Foods
The ascorbic acid and citric acid contents of “bell” peppers increased with gibberellic acid treat-ment,30 whereas in another study the ascorbic acid content of bell peppers was not affected by ethylene treatment.31
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subsequent canning process.23 Jalapeno peppers that were blanched prior to pasteurization lost 75%
of their ascorbic acid,20 whereas a 40% loss of ascorbic acid occurred during water blanching of green bell peppers, and a 15% loss occurred during steam blanching.39 In another study, bell peppers blanched in water lost 24% of their ascorbic acid, though microwave blanched peppers lost only 15%.40 Ascorbic acid content of unblanched “yellow banana” peppers declined substantially during pasteurization and storage, with only 10% remaining after 124 d.41 Calcium chloride brine treatment did not affect ascorbic acid retention in pasteurized yellow banana peppers. In contrast, initial ascorbic acid levels were retained in jalapeno peppers after blanching and pasteurization.42
Blanching may also affect ascorbic acid retention in frozen peppers. Unblanched “padron”
peppers lost 97% of their ascorbic acid within 1 month of freezing, whereas blanching resulted in a 28% loss, followed by an additional 10% loss after 12 months frozen storage.43 In another study, average ascorbic acid losses of ten pepper cultivars that were blanched and stored for 12 months at –12°C (10°F) were 63%, while unblanched cultivars lost 71%.44 Differences in ascorbic acid losses in these studies may be attributed to differences in pepper genetics, brine composition, blanching method, and pasteurization time and temperature.
Ascorbic acid content of dehydrated peppers is influenced by blanching and drying methods.
Paprika fruit lost 63% of its ascorbic acid content when naturally dried, whereas losses of 4 to 54% were observed when freshly harvested and overripe fruit were dried using a forced-air method.45 Other processing parameters may also influence ascorbic acid retention. A 40% loss of ascorbic acid in paprika powder was noted after centrifugation prior to drying, and a 73%
loss occurred in carmelized paprika.46 In another study, drying time and temperature did not affect the ascorbic acid content of dehydrated green bell peppers, but after 8 weeks of storage, blanched peppers dried for 8 h at 60°C (140°F) contained less ascorbic acid than unblanched peppers.47 Unblanched peppers dried for 12 h at 49°C (120°F) contained more ascorbic acid than blanched peppers.
IV. FLAVONOIDS
Pepper fruit are particularly rich in flavonoids, a large class of compounds ubiquitous in plants, that exhibit antioxidant activity, depending on the number and location of hydroxyl groups present.48 In addition to antioxidant function, flavonoids are reported to possess numerous biological, phar-macological, and medicinal properties, including vasodilatory, anticarcinogenic, immune-stimulat-ing, antiallergenic, antiviral, and estrogenic effects, as well as inhibition of various enzymes involved in carcinogenesis.49 In addition, many epidemiological studies indicate an inverse association between the intake of flavonols and flavones and the risk of coronary heart disease,50–52 stroke,53 and lung cancer.54–55
Much progress has been made over the past decade in the identification and quantification of flavonoids and phenolic acids in capsicum fruit due to advancements in HPLC, HPLC-mass spectrometry, and NMR techniques. Suskrano and Yeoman56 identified three hydroxycinnamic acid derivatives: p-coumaroyl, caffeoyl, and 3,4-dimethoxycinnamoyl glycosides, and four flavonoid compounds, although only two were identified: quercetin 3-O-rhamnoside and luteolin 7-O-gluco-side. Iorizzi and colleagues57 identified three hydroxycinnamic acids in Capsicum annuum L. var.
acuminatum fruit, cis-p-coumaric acid-β-D-glucoside, trans-sinapoyl β-D-glucoside, and vanilloyl β-D-glucoside, as well as one flavonoid, quercetin 3-O-rhamnoside. They also identified a unique lignan glycoside (icariside E5) that possesses antioxidant properties. Materska and colleagues58 identified nine compounds in pericarp tissue of hot pepper fruit (Capsicum annuum L., var.
Bronowicka Ostra). The compounds identified included 3 hydroxycinnamic acid derivatives: trans-p-feruloylalcohol-4-)-(6-(2-methyl-3-hydroxypropionyl) glucoside, trans-p-feruoyl-β-D-glucoside, and trans-p-sinapoyl-β-D-glucoside, as well as six flavonoids: luteolin-7-O-(2-apiosyl-4-glucosyl-6-malonoyl)-glucoside, quercetin 3-O-α-L-rhamnoside-7-O-β-D-glucoside, luteolin 6-C-β-D-glu-coside-8-C-α-L-arabinoside, apigenin 6-C-β-D-glucoside-8-C-L-arabinoside, luteolin
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D-apiosyl)-β-D-glucoside], and quercetin 3-O-α-L-rhamnoside. In a subsequent study, Materska and Perucka59 evaluated 4 cultivars of Capsicum annuum L. fruit for phenolic content and antiox-idant capacity and reported that sinapoyl and feruoyl glucosides were the predominant components in red pepper, ranging in concentration from 32 to 42 mg/100 g dry weight, and 15 to 36 mg/100 g dry weight, respectively, whereas quercetin 3-O-L-rhamnoside was the major component in green pepper, ranging in concentration from 33 to 99 mg/100 g dry weight. The antioxidant capacities evaluated by the β-carotene-linoleic acid and DPPH systems correlated highly with phenolic content in the fraction containing phenolic acids and flavonoids. Marin and colleagues24 conducted a detailed characterization of sweet pepper phenolics (Capsicum annuum L., cv. Vergasa), and reported five hydroxycinnamic acid derivatives and 25 flavonoids in pericarp tissue. In addition to the hydrox-ycinnamic acid derivatives and flavonoids, previously identified by Materska and colleagues58 in hot peppers, they identified several novel compounds including 4 flavonoid O-glycosides: luteolin (2-apiosyl-6-acetyl) glucoside, chrysoeriol (2-apiosyl-6-acetyl) glucoside, luteolin 7-O-(2-apiosyl-di-acetyl) glucoside, and luteolin 7-O-2-apiosyl-6-malonyl) glucoside. Additionally, 12 flavonoid glycosides were identified, which included 2 acylated derivatives, luteolin 6-C-(6-malo-nyl)-hexoside-8-C-hexoside, and luteolin 6-C-(6-malony)-hexoside-8-C-pentoside. Quercetin 3-O-rhamnoside and luteolin 7-O-(2-apiosyl-6-malonyl) glucoside were the predominant flavonoids present in red fruit, showing concentrations of 0.31 mg/100 g fresh weight and 0.39 mg/100 g fresh weight, respectively. The concentrations of total hydroxycinnamic acids and total flavonoids in red fruit were 0.44 mg/100 g fresh weight and 2.54 mg/100 g fresh weight, respectively.
The flavonoid content of different pepper types and cultivars is shown in Table 9.2.60 Peppers contain both quercetin (a flavonol) and luteolin (a flavone). Quercetin has a hydroxyl group at C-3 in the aromatic ring, while luteolin does not (see Figure 9.1). The structural differences are important since the presence of a hydroxyl group at C-3 is reported to result in greater free radical-scavenging efficiency.48 In plant cells, flavonoids occur as glycosides, with sugars bound typically at the C-3 position. Flavonoids are commonly quantified in the aglycone form after acid hydrolysis. Flavonoid levels vary greatly among pepper types and cultivars with total levels ranging from 1 to 852 mg/kg.
Interestingly, C. annuum cultivars contain higher levels of flavonoids than C. chinense cultivars. Low levels of flavonoids in the pungent C. chinense peppers may indicate diversion of phenolic precursors from flavonoid to capsaicinoid synthesis. An exception is the C. frutescens cv. tabasco, which contains much higher levels of luteolin than the other Capsicum species and cultivars. It appears that fruit from different Capsicum species vary greatly in their genetic capacity for synthesizing specific flavonoids. Plant breeders and molecular biologists may take advantage of this genetic variability to increase the flavonoid content of Capsicum fruit. The exceptionally high flavonoid levels reported by Lee and colleagues18 may be due to differences in genetics and environmental conditions in which the peppers were grown. Environmental stress during plant growth has been shown to stimulate the phenylpropanoid pathway and production of various phenolic compounds.
Increasing luteolin levels in pepper fruit may be important for prevention of coronary heart disease. A luteolin-rich artichoke extract was recently shown to protect LDL from oxidation in vitro, which may be due to its antioxidant function or ability to sequester prooxidant metal ions.61Additionally, luteolin does not complex with copper ions to produce oxidative damage to DNA, which contrasts with the prooxidant effect observed for quercetin.62
Total flavonoid content of pepper cultivars generally declines as fruit ripens and changes color.
For instance, immature green pepper of sweet peppers (Capsicum annuum L.) cv. Vergasa had a very high phenolic content, but green, immature red, and red ripe peppers showed a four- to fivefold reduction.24 Red fruit generally contain higher levels of hydroxycinnamic acids than green fruit, whereas green fruit contain higher levels of flavonoids than red fruit.21,24,59 However, exceptions to this rule include the cayenne cv. Mesilla, in which the flavonoid content increased during maturation, and the “long yellow” cv. Inferno, and tabasco cv. Tabasco, in which no change in flavonoid content occurred during ripening.21 In terms of antioxidant capacity, red fruit generally have greater radical scavenging capacity than green fruit, 21,57,59 which may be attributed to higher levels of
hydroxy-Antioxidant Vitamin and Phytochemical Content of Fresh and Processed Pepper Fruit 173
cinnamic acid glycosides and capsaicinoids in the ripe fruit.59 The loss of flavonoids observed during ripening of most cultivars is consistent with reported flavonoid losses that occur during maturation of C. frutescens fruit.56 Flavonoid losses during ripening may reflect metabolic conversion of flavonoids to secondary phenolic compounds.63 The oxidoreductase enzymes polyphenol oxidase64 and peroxidase65,66 may play a role in degradation of flavonols during maturation and senescence.